1. Field of the Invention
The present invention relates to a voltage nonlinear resistor.
2. Description of the Related Art
In an electric power facility such as a substation, an overvoltage protection device such as a lightning arrester and a surge absorber is installed to protect a switchgear, a transformer, and the like from abnormal voltage caused by lightning or the like. In such an overvoltage protection device, a voltage nonlinear resistor exhibiting low resistance at the time of receiving abnormal voltage is used. As the voltage nonlinear resistor, a zinc oxide element (ZnO element) consisting essentially of zinc oxide (ZnO) and containing at least one additive is mainly used.
In recent years, for the purpose of facility miniaturization and cost reduction, the overvoltage protection device is required to be miniaturized. Along with this, the zinc oxide element used in the lightning arrester is also required to be miniaturized.
To miniaturize the zinc oxide element, consideration of an increase in operating start voltage, that is, varistor voltage, of the voltage nonlinear resistor is required. Also, when the zinc oxide element is miniaturized, the amount of electric energy to be applied per unit volume increases. Thus, an energy withstand characteristic and a voltage clamping ratio of the element need to be improved.
JP-10-270209-A considers improvement of the varistor voltage and the voltage clamping ratio. JP-10-270209-A discloses a voltage nonlinear resistor of a sintered substance of a composite consisting essentially of zinc oxide and containing a plurality of rare earth elements, at least one of which is selected from the group consisting of Eu, Gd, Tb, Dy, Ho, Y, Er, Tm, Yb, and Lu, and Bi and Sb, wherein spacing do (Å) between grains of the zinc oxide, provided from precipitation grains formed in the grains of the zinc oxide or on a grain boundary lies in the range of 2.85 Å≤d1≤2.91 Å, 1.83 Å≤d2≤1.89 Å, 1.77 Å≤d3≤1.82 Å, 1.56 Å≤d4≤1.61 Å, and 1.54 Å≤d5≤1.60 Å. The above literature is supposed to exert an effect of providing a voltage nonlinear resistor with large varistor voltage and a small large current area voltage clamping ratio.